go/constant: add examples for working with constant.Value
go/constant: add examples for working with constant.Value

Updates #24352.

Change-Id: Ie7c1de905154a483b7f7748de28e35b484cce6ea
Reviewed-on: https://go-review.googlesource.com/101279
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

akutz/go,CAFxX/go,Cofyc/go,Cofyc/go,nawawi/go,nawawi/go,codestation/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,Cofyc/go,josharian/go,josharian/go,josharian/go,Cofyc/go,josharian/go,golang/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,codestation/go,nawawi/go,josharian/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,Cofyc/go,codestation/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,golang/go,Cofyc/go,akutz/go,codestation/go,golang/go,akutz/go,golang/go,josharian/go,Cofyc/go,codestation/go,codestation/go,josharian/go,akutz/go,CAFxX/go,josharian/go,Cofyc/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,golang/go,codestation/go,codestation/go,nawawi/go